Does vamp A's controller have an opportunity at this point to use Heidelberg Castle?

Name: Siren's Lure
[BL:R1, LoB:R]
Cardtype: Action Modifier
Discipline: Melpominee/Celerity
[cel] This vampire burns 2 blood to get +1 stealth.
[mel] Only usable by an unlocked ready vampire other than the acting minion when a minion attempts to block. That minion cannot block this action. Lock this vampire and that minion. Once the action resolves, this vampire enters combat with that minion.
[MEL] As [mel] above, but do not lock this modifying vampire.
Artist: Steve Prescott